1. Field of the Invention
The present invention relates generally to Digital Color Modulation for interfacing digital logic with a TV set's Radio-Frequency (RF) analog input. More specifically, the present invention relates to a Digital Color Modulator which interfaces between digital logic and the RF input of a conventional TV set adapted for providing well defined color characters and symbols on a dot-by-dot basis.
2. Description of the Prior Art
In 1953 the National Television System Committee (NTSC) established specifications for color television signals. They specified that color defining signals (chrominance signals) would modulate a subcarrier that is approximately 3.58 MHz above brightness-modulated carriers (luminance signals). This permits the separation of the two signals by detectors in a TV set and maintains the compatibility of this new color standard with existing black and white sets. Specifically, black and white sets detect only the luminance signal. In contrast, color TV sets first decode the luminance signal to determine how much light is at a certain point on the screen, then decode the chrominance signal for determination of how much of that light is red, blue and green. However, the definition of a high-resolution pattern of rapidly changing colors requires a high frequency signal, which in turn requires a wide bandwidth. The bandwidth of each TV channel is limited to about 6 MHz, and the chrominance signal is restricted to a 2 MHz bandwidth. Fast color changes over small areas cannot be transmitted in a 2 MHz bandwidth. However, the human eye is unable to perceive color in small viewed areas and the brain "fills in" color from a surrounding area into the small areas. Thus, while smaller areas are transmitted essentially in black and white, the brain "fills in" the proper color, resulting in adequate resolution for color TV graphics being attainable within the bandwidth limitations of the chrominance signals.
Existing video cameras intrinsically provide a considerable degree of spatial filtering which limits the high frequency content of the luminance and chrominance signals. However, when it is desired to interface digital circuits with the NTSC standard, the sudden digital transitions associated with digital signals, which have large high-frequency components, cause cross-channel modulation between the luminance and chrominance signals. Merely low-pass filtering the digital signal is inadequate as this results in a loss of sharp contrast and luminance consistency which are desirable characteristics for the characters and symbols required for TV graphics. Accordingly, it is desirable to have a digital interface for an analog NTSC receiver which provides sharp contrast and luminance consistency for TV graphics.